The political pressure on FBI director James Comey intensified on Saturday night as four powerful Democratic senators demanded immediate answers about the bureau’s announcement it is examining new material as part of its investigation about Hillary Clinton’s email server.

Patrick Leahy of Vermont, Dianne Feinstein of California, Thomas Carper of Delaware and Benjamin Cardin of Maryland wrote to Comey and the attorney general, Loretta Lynch, to insist on a thorough briefing by Monday about Comey’s decision to tell congressional officials about new material from an “unrelated case” – a decision that shook the campaigns with only 10 days to go.

“[N]o later than Monday 31 October 2016,” the senators wrote, “we request you provide us with more detailed information about the investigative steps being taken, the number of emails involved, and what is being done to determine how many of the emails are duplicative of those already reviewed by the FBI.”

A day after Comey released a vague letter informing officials that new material from an “unrelated case” had prompted the FBI to revisit the Clinton email investigation, he was directly criticised by the Clinton campaign, surrogates and others for inserting the bureau into the election.

Lynch and deputy Sally Yates had recommended that Comey not tell Congress about the new review, out of deference to longstanding policy against real or perceived interference in politics in the days before a vote.

Legal observers were perplexed by Comey’s Friday letter to Congress, particularly after he subsequently told FBI employees “we don’t know the significance of this newly discovered collection of emails”, apparently from a device jointly used by Weiner and his estranged wife, Clinton aide Huma Abedin.

Comey and his allies have argued the director found himself in an impossible position.

After recommending in July against indicting Clinton for being “extremely careless” with classified information contained on the server she used while secretary of state, Comey testified to dissatisfied congressional Republicans that revisiting the decision would be unlikely.

Now, he feared the political fallout of staying silent about making a reversal by reopening the matter before the election.

Once Comey opted to send his letter, Republican presidential nominee Donald Trump, who has pledged to jail Clinton if elected, claimed vindication.

The Democratic senators’ response also underscores the precariousness of Comey’s own position.

Should the Democrats win a majority of Senate seats next month, Feinstein is likely to chair the intelligence committee, Leahy the judiciary committee and Carper the homeland security and government affairs committee. Comey frequently testifies before all three.

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Cardin is likely to chair the foreign relations committee. Should the Republicans maintain control, all will be senior ranking members of panels on which the FBI relies for political and budgetary support.

The four senators called Comey’s letter “troubling”, both in its vagueness and for “break[ing] with the longstanding tradition of Department of Justice and FBI of exercising extreme caution in the days leading to up an election, so as not to unfairly influence the results”.

They called into question whether the FBI had “even obtained or reviewed” material from the Weiner device, implying that Comey had prioritized his political position ahead of the facts of the case.

Challenging Comey to begin the fateful final full week of the election campaign with additional disclosures, the senators wrote that to do otherwise would be “irresponsible and a disservice to the American people”.